Light-emitting element, light-emitting device, lighting device, and electronic devices

ABSTRACT

A light-emitting element which at least includes a monomolecular layer including a luminescent center material with a fluorescent light-emitting property, and a monomolecular layer including a host material with a carrier (electron or hole)-transport property and a band gap larger than a band gap (note that a band gap refers to the energy difference between a HOMO level and a LUMO level) of the luminescent center material, between a pair of electrodes, in which the monomolecular layer including the host material and the monomolecular layer including the luminescent center material share the same interface, is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element, alight-emitting device, a lighting device, and an electronic device, eachof which utilizes electroluminescence of an organic material.

2. Description of the Related Art

An organic compound can take a wider variety of structures than aninorganic compound, and have a possibility to synthesize a materialhaving various functions by appropriate molecular-design of an organiccompound. Owing to those advantages, electronics utilizing a functionalorganic material has been attracting attention in recent years.

For example, a solar cell, a light-emitting element, an organictransistor, and the like are exemplified as electronic devices utilizingan organic compound as a functional material. These are devices takingadvantage of electric properties and optical properties of the organiccompound. Among them, in particular, a light-emitting element has beenmaking remarkable development.

The light emission mechanism of a light-emitting element is as follows:electrons injected from a cathode and holes injected from an anoderecombine in the luminescent center of a light-emitting layer to formmolecular excitons by applying a voltage between a pair of electrodeswith the light-emitting layer interposed therebetween; and when themolecular excitons relax to a ground state, energy is released to emitlight.

Here, it is said that the internal quantum efficiency of alight-emitting element depends on the recombination efficiency, theexciton generation efficiency, the quantum efficiency, and the like ofcarriers.

Among them, in the exciton generation efficiency, quantum physicalchemistry shows that the ratio of a triplet exciton and a singletexciton, which are generated, is statistically 3:1. For this reason, themaximum internal quantum efficiencies of a light-emitting element usinga fluorescent light-emitting material and a light-emitting element usingphosphorescent light-emitting material can be 0.25 and 1, respectively.

The recombination efficiency shows the probability of recombination ofcarriers which are injected from one electrode and do not pass throughthe other electrode. In order to improve the recombination efficiency, ablock layer for trapping carriers is provided with respect to holes orelectrons in some cases.

The internal quantum efficiency is defined by the rate of generatedphotons with respect to the number of electrons injected to alight-emitting element. An improvement of the internal quantumefficiency improves luminous efficiency and reduces power consumption;therefore, many researches have been made in aspects of a material andan element structure (for example, see Patent Document 1).

[Reference]

-   [Patent Document 1] Japanese Published Patent Application No.    2004-342383

SUMMARY OF THE INVENTION

Deactivation of excitation energy is given as a cause of a decrease inthe internal quantum efficiency. When energy of carriers recombined in ahost cannot be effectively conveyed to a guest and the energy isdeactivated, a large amount of energy is lost, resulting in a decreasein the internal quantum efficiency.

In view of the above, an object of the present invention is to provide alight-emitting element with high luminous efficiency, in which energytransfer from a host to a guest is well.

In addition, the penetration of carriers is also a significant cause ofa decrease in the internal quantum efficiency. The “penetration ofcarriers” means that carriers pass through a light-emitting layerwithout recombining. Carriers that have passed through a light-emittinglayer without recombining do not contribute to light emission, so thatthe carriers flow as a wasted current. However, when a carrier blocklayer is used in order to prevent the wasted current, an inconveniencesuch as an increase in a driving voltage occurs.

In view of the above, an object of the present invention is to provide alight-emitting element with favorable luminous efficiency, in which thepenetration of carriers is effectively prevented, or a light-emittingelement with favorable luminous efficiency, in which carrier transferfrom a host to a guest is well.

Further, an object of the present invention is to provide alight-emitting device or a lighting device with low power consumption.

Furthermore, an object of the present invention is to provide anelectronic device with low power consumption.

Note that one embodiment of the present invention may achieve at leastone of the above-described objects.

In view of the above objects, the present inventors have found that in ageneral host-guest type light-emitting layer, a guest is dispersed in ahost; thus, loss of energy transfer occurs.

One embodiment of the present invention is a light-emitting elementwhich at least includes a monomolecular layer including a luminescentcenter material with a fluorescent light-emitting property, and amonomolecular layer including a host material with a carrier (electronor hole)-transport property and a band gap larger than a band gap of theluminescent center material, between a pair of electrodes (Note that aband gap refers to the energy difference between a HOMO level and a LUMOlevel), and in which one surface of the monomolecular layer includingthe host material is in contact with one surface of the monomolecularlayer including the luminescent center material.

Further, one embodiment of the present invention is a light-emittingelement which at least includes a monomolecular layer including aluminescent center material with a phosphorescent light-emittingproperty, and a monomolecular layer including a host material with acarrier-transport property and a triplet excited energy larger than atriplet excited energy of the luminescent center material, between apair of electrodes, and in which one surface of the monomolecular layerincluding the host material is in contact with one surface of themonomolecular layer including the luminescent center material.

The present inventors have found that the internal quantum efficiency isimproved with a structure in which excitation energy recombined in ahost is more effectively transferred to a guest.

That is, one embodiment of the present invention is a light-emittingelement with the above structure, in which a skeleton that contributesto donation of excitation energy in the host material is adjacent to askeleton that contributes to acceptance of excitation energy in theluminescent center material.

A structure with which a carrier is easily injected to a monomolecularlayer of the host material and a monomolecular layer of the guestmaterial is also effective.

That is, one embodiment of the present invention is a light-emittingelement with the above structure, in which a layer including a compositematerial is further included between the pair of electrodes. In thecomposite material, an acceptor substance is mixed in a substance with ahigh hole-transport property. A monomolecular layer of the host materialis in contact with the layer including the composite material at aninterface different from an interface shared by the monomolecular layerof the host material and a monomolecular layer of the luminescent centermaterial.

When a plurality of monomolecular layers of the host material and aplurality of monomolecular layers of the guest material are formed, theleakage of carriers can be reduced. Further, by further providing alayer including the composite material, an increase in a driving voltagedue to stacking of the layers can be suppressed.

That is, one embodiment of the present invention is a light-emittingelement with the above structure, in which a plurality of stacks isincluded, each of the stacks including the layer including the compositematerial, the monomolecular layer of the luminescent center material,and the monomolecular layer of the host material.

The light-emitting elements such that described in the above have highluminous efficiency; therefore, a light-emitting device or a lightingdevice including a light-emitting element with the above structure haslow power consumption.

Thus, one embodiment of the present invention is a light-emitting deviceor a lighting device including the above light-emitting element and acontrol circuit controlling light emission of the light-emittingelement.

Note that the light-emitting device in this specification includes imagedisplay devices, light-emitting devices, or light sources (includinglighting device). Further, the light-emitting device includes all of thefollowing modules: modules in which a connector such as an FPC (FlexiblePrinted Circuit), a TAB (Tape Automated Bonding) tape, or a TCP (TapeCarrier Package) is attached to a panel in which a light-emittingelement is formed; modules having a TAB tape or a TCP provided with aprinted wiring board at the end thereof; and modules having an IC(Integrated Circuit) directly mounted on a light-emitting device by aCOG (Chip On Glass) method.

An electronic device using the light-emitting element of the presentinvention in its display portion is also included in the category of thepresent invention. Therefore, an electronic device of the presentinvention includes a display portion or a lighting portion including theabove light-emitting element and the above control circuit controllinglight emission of the light-emitting element.

The light-emitting element of the present invention has high luminousefficiency.

The light-emitting device or the lighting device of the presentinvention has low power consumption.

The electronic device of the present invention has low powerconsumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are conceptual views of the present invention.

FIG. 2 is a conceptual view of the present invention.

FIGS. 3A and 3B each show a light-emitting element of the presentinvention.

FIG. 4 shows a light-emitting element of the present invention.

FIGS. 5A and 5B show a light-emitting device of the preset invention.

FIGS. 6A and 6B show a light-emitting device of the present invention.

FIGS. 7A to 7D each show an electronic device of the present invention.

FIGS. 8A to 8C each show an electronic device of the present invention.

FIG. 9 shows an electronic device of the present invention.

FIG. 10 shows an electronic device of the present invention.

FIG. 11 shows a lighting device of the present invention.

FIG. 12 shows a lighting device of the present invention.

FIG. 13 shows lighting devices of the present invention.

FIG. 14 shows the direction of a transition dipole moment of CzPA.

FIGS. 15A and 15B show HOMO and LUMO of CzPA.

FIG. 16 shows the direction of a transition dipole moment of 1,6DPhAPrn.

FIGS. 17A and 17B show HOMO and LUMO of 1,6DPhAPrn.

FIG. 18 shows the direction of a transition dipole moment and thedirection of polarization of light emission.

FIG. 19 shows the ideal arrangement of CzPA and 1,6DPhAPrn in view ofgiving and receiving of energy.

FIGS. 20A and 20B each show the favorable direction of a transitiondipole moment of a host material and a luminescent center material.

FIG. 21 shows the ideal arrangement related to the position of asubstrate and the direction of a transition dipole moment of lightemission of the luminescent center material.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings. Note that the invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the invention.Therefore, the invention should not be construed as being limited to thedescription in the following embodiments.

(Embodiment 1)

The present inventors have considered that transfer of excitation energyin a host by carriers recombined in the host to a guest affects theinternal quantum efficiency of a current-excitation type light-emittingelement using an organic compound as a luminescent center substance. Ina light-emitting element including a host-guest type light-emittinglayer, a guest that is a luminescent center substance is dispersed in ahost material that is to be a body. Therefore, part of energy ofcarriers recombined in the host has been deactivated before it has beentransferred to the guest, resulting in a decrease in the internalquantum efficiency.

Further, the present inventors have considered that in the case where aguest is dispersed in a host material, part of carriers injected to thehost material are not recombined in the guest and penetrate to anadjacent layer.

Here, in a light-emitting element described in this embodiment, guestmolecules that are luminescent center materials exist in contact withall the host molecules in a light-emitting layer. In the light-emittingelement having such a structure, carriers injected to the host moleculeor excitation energy generated in the host molecule can be effectivelytransferred to the luminescent center material.

That is, when a monomolecular layer of a host material and amonomolecular layer of a luminescent center material are overlapped witheach other, in other words, one surface of the monomolecular layer ofthe host material and one surface of the monomolecular layer of theluminescent center material are in contact with each other, excitationenergy of the host can be effectively transferred to the luminescentcenter material as described above, whereby the internal quantumefficiency can be improved and a light-emitting element with highluminous efficiency can be obtained.

In that case, in order to smoothly give and receive excitation energy,it is preferable that a skeleton having a function of giving excitationenergy of the host material (luminophore) and a skeleton having afunction of receiving excitation energy of the luminescent centermaterial (chromophore) be adjacent to each other.

Further, in order to smoothly give and receive excitation energy, it ispreferable that a skeleton having a function of giving carriers in thehost material and a skeleton having a function of receiving carriers inthe luminescent center material be adjacent to each other.

As a combination of a host material and a luminescent center material,which can have such an arrangement, a combination of materials in whichpart of a skeleton of a host material and part of a skeleton of aluminescent center material are similar or the same is given. Forexample, when a host material and a luminescent center material, eachhaving a skeleton formed of a planar condensed ring are formed, thecondensed ring portions are easily formed to be adjacent to each other.Then, carriers or excitation energy can be effectively given andreceived through the condensed ring portions. Similarly, in the case ofa host material and a luminescent center material, each having aheterocyclic ring skeleton, the heterocyclic ring skeleton portions ofthe host material and the luminescent center material are easily formedto be adjacent to each other. Thus, carriers or excitation energy can beeffectively given and received through the heterocyclic ring skeletonportions. Further, a host material having two nitrogen atoms such as aquinoxaline skeleton is similar to an iridium complex having a molecularincluding two nitrogen atoms as a ligand in that a heterocyclic ringskeleton having two nitrogen atoms is included. Portions of the hostmaterial and the iridium complex, which are similar to each other, areeasily formed to be adjacent to each other; therefore, carriers orexcitation energy can be effectively given and received from the hostmaterial to the luminescent center material through the portions.

As another method for effectively giving and receiving excitation energyfrom a host material to a luminescent center material, there is a methodin which the direction of a transition dipole moment of light emissionof the host material and the direction of a transition dipole moment ofelectronic transition of the luminescent center material are arranged tobe the same direction as much as possible. A “transition dipole moment”is a difference in polarization of charges when transition between twostates occurs. Note that these transition dipole moments can be obtainedby the time-dependent density functional theory.

A light-emitting element having the above-described structure isdescribed with reference to the drawings.

FIG. 1A is a conceptual view showing a stacked layer in which onesurface of a monomolecular layer of a host material 300 is in contactwith one surface of a monomolecular layer of a luminescent centermaterial 301. The light-emitting element described in this embodiment isa light-emitting element including such a stack in a light-emittinglayer. Since the host material and the luminescent center material arein contact with each other and the luminescent center material exists ina film form, not in a dispersed form, carriers recombined in the hostmaterial or excitation energy can be effectively transferred to theluminescent center material; consequently, the internal quantumefficiency is improved.

Such a monomolecular layer can be formed by relatively transferring asurface where the monomolecular layer is to be manufactured with respectto an evaporation source of the host material or the luminescent centermaterial at appropriate speed. In order to manufacture a stackedstructure, one monomolecular layer may be formed by the above method andthen the other monomolecular layer may be formed in a similar mannerusing a different material. Further, an existing method such as a methodusing a self-assembled monomolecular layer (a SAM film), aLangmuir-Blodgett film (a LB film), or the like may be used.

Further, it is preferable that the monomolecular layer of the hostmaterial and the monomolecular layer of the luminescent center materialbe alternately stacked to form a stack. At this time, a monomolecularlayer of the host material and a monomolecular layer of the luminescentcenter material, which are adjacent to each other, share the sameinterface. In the case of forming a stack, when focused on an n-thmonomolecular layer of the luminescent center material existed in thestack, one interface is shared with an n-th monomolecular layer of thehost material and the other interface is shared with an (n+1)-thmonomolecular layer of the host material. Similarly, when focused on the(n+1)-th monomolecular layer of the host material, one interface isshared with the n-th monomolecular layer of the luminescent centermaterial and the other interface is shared with the (n+1)-thmonomolecular layer of the luminescent center material.

The total thickness of the stack is preferably greater than or equal to30 nm and less than or equal to 200 nm. This is because carrier balanceis easily achieved when the total thickness is greater than or equal to30 nm and a driving voltage suitable for a light-emitting element iseasily kept when the total thickness is less than or equal to 200 nm.

Further, it is more preferable for the monomolecular layer to have asuperlattice structure.

In the host material 300 in FIG. 1B, a portion denoted by T shows askeleton contributed to donation of carriers or excitation energy in thehost material. A portion denoted by H_(R) shows a skeleton having theother functions, and corresponds to a portion that receives carriers orexcitation energy, for example; however, the functions of the portionsare not particularly limited, and T and H_(R) can be the same skeletonsin some cases. Similarly, in the luminescent center material 301, aportion denoted by G_(R) shows a skeleton contributed to acceptance of acarrier or excitation energy in the luminescent center material. Aportion denoted by E shows a skeleton having the other functions, andcorresponds to a skeleton having a function of light emission, forexample; however, the functions of the portions are not particularlylimited, and G_(R) and E can be the same skeletons in some cases.

As shown in FIG. 1B, a light-emitting element includes a structure inwhich a skeleton H_(R) donating carriers or excitation energy of a hostmaterial and a skeleton G_(R) accepting carriers or excitation energy ofa luminescent center material are formed to be adjacent to each other ina light-emitting layer. With this light-emitting element, carriersrecombined in the host material or excitation energy can be effectivelytransferred to the luminescent center material, whereby the internalquantum efficiency can be improved, which leads to an improvement of theluminous efficiency of the light-emitting element.

As a combination of a host material and a luminescent center materialwhich can have such an arrangement, a combination of materials in whichpart of a skeleton of a host material and part of a skeleton of aluminescent center material are similar or the same can be given. Forexample, it is considered that when a host material and a luminescentcenter material, each having a skeleton formed of a planar condensedring are formed, the condensed ring portions are easily Ruined to beadjacent to each other. Then, carriers or excitation energy can beeffectively given and received through the condensed ring portions.Examples of such a skeleton formed of a planar condensed ring are ananthracene skeleton, a pyrene skeleton, a triphenylene skeleton, and thelike.

Similarly, in the case of a host material and a luminescent centermaterial, each having a heterocyclic ring skeleton, it is consideredthat the heterocyclic ring skeleton portions of the host material andthe luminescent center material are easily formed to be adjacent to eachother; therefore, carriers or excitation energy can be effectively givenand received through the heterocyclic ring skeleton portions.

Further, it is considered that a host material having two nitrogen atomssuch as a quinoxaline skeleton is similar to an iridium complex having,as a ligand, a molecular including two nitrogen atoms in that aheterocyclic ring skeleton having two nitrogen atoms is included. It isconsidered that the portions of the host material and the iridiumcomplex, which are similar to each other, are easily formed to beadjacent to each other. Thus, carriers or excitation energy can beeffectively given and received from the host material to the luminescentcenter material through the portions.

In order that carriers are easily transferred from the host material tothe luminescent center material, it is preferable that the direction ofa plane surface of a condensed ring or a heterocyclic ring (thedirection of π conjugation) in the host be the same as the direction ofa plane surface of a condensed ring or a heterocyclic ring (thedirection of π conjugation) in the guest as much as possible. Further,in that case, it is preferable that the condensed ring or theheterocyclic ring be a substituent with a lowest excitation level in amolecule.

Note that when a guest material is excited in a state where a planesurface of the condensed ring of the guest is overlapped with a planesurface of a condensed ring of a material other than the guest material(a state of stacking) or in a state where a plane surface of theheterocyclic ring of the guest is overlapped with a plane surface of aheterocyclic ring of a material other than the guest material (a stateof stacking), the guest material and the other material might form anexciplex. In that case, an exciton with more stable level than that ofexcitation energy of the original guest material is formed and lightemission having a wavelength longer than the emission spectrum of theoriginal guest material might be obtained.

Therefore, when light emission only from the guest material is required,even if the condensed rings are adjacent to each other, the condensedrings are preferably designed so as not to overlap with each other inorder that the condensed rings do not become stacking. For that purpose,an appropriate substituent may be introduced to these condensed rings.

Such a stack of the host material and the luminescent center materialcan be manufactured in a manner similar to that described in FIG. 1A.

In FIG. 1C, a layer formed of a composite material 302 in which anacceptor substance is included in a substance with a high hole-transportproperty is further provided over the stack of the monomolecular layerof the host material 300 and the monomolecular layer of the luminescentcenter material 301, in order to improve a carrier injection property tothe stack. With such a structure, carriers are smoothly injected to thestack, which contributes to a reduction in a driving voltage of thelight-emitting element.

Note that in this specification, a “composition material” does notsimply refer to a material in which two materials are mixed but refersto a material in a state where charges can be given and received betweenmaterials by mixing a plurality of materials. In some cases, the givingand receiving of charges includes giving and receiving of chargesrealized only in the case where there is an auxiliary effect of anelectric field.

As the acceptor substance that can be used in the composite material,organic compounds such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like are given. Further, a transition metaloxide can be given. Furthermore, an oxide of metals that belong to Group4 to Group 8 of the periodic table can be given. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferablebecause of their high electron-accepting properties. Among these,molybdenum oxide is especially preferable because it is stable in theair and is easily treated because of its low hygroscopic property.

As the substance having a high hole-transport property used for thecomposite material, various compounds such as an aromatic aminecompound, a carbazole compound, aromatic hydrocarbon, and a highmolecular compound (such as an oligomer, a dendrimer, and a polymer) canbe used. An organic compound used for the composite material ispreferably an organic compound having a high hole-transport property.Specifically, a substance having a hole mobility of higher than or equalto 10⁻⁶ cm²/Vs is preferably used. However, any substance other than theabove substances may also be used as long as it has a higherhole-transport property than an electron-transport property. An organiccompound, which can be used as a substance having a high hole-transportproperty for the composite material, will be specifically given below.

As aromatic amine compounds, for example, there areN,N′-di(p-tolyl)-N,N-diphenyl-p-phenylenediamine (abbreviation: DTDPPA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Examples of the carbazole compound which can be used for the compositematerial include3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

In addition, examples of the carbazole compound which can be used forthe composite material include 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbon which can be used for the compositematerial include 2-tert-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA); 2-tert-butyl-9,10-di(1-naphthyl)anthracene;9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA);2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA);9,10-di(2-naphthyl)anthracene (abbreviation: DNA);9,10-diphenylanthracene (abbreviation: DPAnth); 2-tert-butylanthracene(abbreviation: t-BuAnth); 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA);2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene;9,10-bis[2-(1-naphthyl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene;2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene; 9,9′-bianthryl;10,10′-diphenyl-9,9′-bianthryl;10,10′-bis(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; anthracene;tetracene; rubrene; perylene, and 2,5,8,11-tetra(tert-butyl)perylene.Besides those, pentacene, coronene, or the like can also be used. Inparticular, the aromatic hydrocarbon which has a hole mobility higherthan or equal to 1×10⁻⁶ cm²/Vs and which has 14 to 42 carbon atoms isparticularly preferable.

The aromatic hydrocarbon which can be used for the composite materialmay have a vinyl skeleton. As the aromatic hydrocarbon having a vinylgroup, the following are given for example:4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi);9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA);and the like.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine (abbreviation:Poly-TPD) can also be used.

FIG. 2 shows a layered structure of a plurality of stacks of thecomposite material 302, the host material 300, and the luminescentcenter material 301 in FIG. 1C. When a plurality of stacks is stacked,carriers that cannot be completely trapped in one stack can be trappedin the next stack, so that carriers penetrating the light-emitting layercan be drastically reduced. Thus, the flow of a wasted current can besuppressed, which leads to an improvement in the internal light emissionefficiency. When forming a stack, a plurality of stacks in which a layerformed of a composite material, a monomolecular layer of a hostmaterial, and a monomolecular layer of a luminescent center material arestacked in this order is stacked. In that case, a monomolecular layer ofthe luminescent center material in an n-th stack and a layer of thecomposite material in an (n+1)-th stack share the same interface. Notethat a layer formed of a material with a carrier-transport property(e.g., a layer of the host material) may be interposed between the n-thstack and the (n+1)-th stack.

Despite fears of an increase in a driving voltage by forming such aplurality of stacks, the layer formed of the composite material 302enables the plurality of stacks to be formed without an extra increasein the driving voltage.

The total thickness of the stacks is preferably greater than or equal to30 nm and less than or equal to 200 nm. This is because carrier balanceis easily achieved when the total thickness is greater than or equal to30 nm and a driving voltage suitable for a light-emitting element iseasily kept when the total thickness is less than or equal to 200 nm.

A light-emitting element with the above-described structure can havehigh internal quantum efficiency and luminous efficiency.

(Embodiment 2)

In this embodiment, 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA) was used for a host material andN,N,N′,N′-tetraphenylpyrene-1,6-diamine (abbreviation: 1,6DPhAPrn) wasused for a luminescent center material, and the orientation of1,6DPhAPrn, by which light emitted from CzPA can be easily absorbed wasanalyzed by the quantum chemistry calculation. Molecular structures ofthe materials are shown in the following structural formulae.

In the calculation, the molecular structures were optimized and then thetransition dipole moment of light emission of the host material and thetransition dipole moment of absorption of the luminescent centermaterial were analyzed. Moreover, a molecular orbital related to thetransition was analyzed.

The density functional theory (DFT) using Gaussian basis was employedfor the structure optimization calculation. The time-dependent densityfunctional theory (TDDFT) was employed for the calculation of thetransition dipole moment. In the DFT, an exchange-correlationinteraction is approximated by a functional (a function of a function)of one electron potential represented in terms of electron density toenable highly speed calculations. Here, B3LYP that is a hybridfunctional was used to specify the weight of each parameter related toexchange-correlation energy. In addition, as a basis set, 6-311G (abasis set of a triple-split valence basis set using three contractionfunctions for each valence orbital) was applied to all the atoms. By theabove basis set, for example, orbitals of 1s to 3s are considered in thecase of hydrogen atoms while orbitals of 1s to 4s and 2p to 4p areconsidered in the case of nitrogen atoms. Furthermore, to improvecalculation accuracy, the p function and the d function as polarizationbasis sets were added to hydrogen atoms and atoms other than hydrogenatoms, respectively, and the p orbital and the d orbital wereconsidered.

Gaussian 09 was used as a quantum chemistry computational program. Ahigh performance computer (manufactured by SGI Japan, Ltd., Altix 4700)was used for the calculations.

The singlet excited state of CzPA was analyzed by the calculation. Thefirst excited state is a transition between the highest occupiedmolecular orbital (HOMO) and the lowest unoccupied molecular orbital(LUMO). A transition dipole moment of light emission related to thetransition is shown in FIG. 14. The transition dipole moment exists on along axis (X-axis) of the molecule and the direction is shown by anarrow in FIG. 14.

The highest occupied molecular orbital (HOMO) and the lowest unoccupiedmolecular orbital (LUMO) in the most stable structure obtained by thecalculation to optimize the structure of CzPA are visualized by GaussView 5.0.8 and shown in FIGS. 15A and 15B. FIG. 15A shows the highestoccupied molecular orbital (HOMO), and FIG. 15B shows the lowestunoccupied molecular orbital (LUMO). The spheres in the figuresrepresent atoms that constitute CzPA, and cloud-like objects around theatoms represent the highest occupied molecular orbital (HOMO) or thelowest unoccupied molecular orbital (LUMO).

From FIGS. 15A and 15B, the highest molecular occupied orbital existsaround an anthracene skeleton in CzPA, so that it is found that ananthracene skeleton largely contributes to the hole-injection propertyof CzPA. In addition, the lowest molecular unoccupied orbital alsoexists around an anthracene skeleton, so that it is found that ananthracene skeleton largely contributes to the electron-transportproperty of CzPA.

Next, the singlet excited state of 1,6DPhAPrn was analyzed by thecalculation. The first excited state was a transition between thehighest occupied molecular orbital (HOMO) and the lowest unoccupiedmolecular orbital (LUMO). A transition dipole moment of absorptionrelated to the transition is shown in FIG. 16. The transition dipolemoment exists on a plane surface (X-Z plane) of a pyrene group and thedirection is shown by an arrow in FIG. 16.

The highest occupied molecular orbital (HOMO) and the lowest unoccupiedmolecular orbital (LUMO) in the most stable structure of 1,6DPhAPrn areshown in FIGS. 17A and 17B. FIG. 17A shows the highest occupiedmolecular orbital (HOMO) and FIG. 17B shows the lowest unoccupiedmolecular orbital (LUMO).

From FIGS. 17A and 17B, the highest occupied molecular orbital existsaround a pyrenyl group or a phenyl group in 1,6DPhAPrn, so that it isfound that the pyrenyl group or the phenyl group largely contributes tothe hole-transport property of 1,6DPhAPrn. In addition, the lowestunoccupied molecular orbital exists around a pyrenyl group, so that itis found that a pyrenyl group largely contributes to theelectron-transport property of 1,6DPhAPrn.

When the direction of the transition dipole moment of a molecule and thedirection of polarization of light are the same, the transitionprobability is maximized and light is absorbed or emitted. Therefore,when the direction of polarization of light emitted from a host materialis the same as the direction of the transition dipole moment ofabsorption of a luminescent center material, light is easily absorbedand the energy transfer efficiency is increased.

Assuming that the transition dipole moment of light emission exists on aY-axis direction as in FIG. 18, light is emitted to a direction in theX-Z plane with electronic transition. The direction of polarization oflight is vertical and light is not emitted to the Y-axis direction.

Consequently, when the directions of the transition dipole moments arearranged in parallel, the energy transfer efficiency between CzPA thatis a host material and 1,6DPhAPm that is a luminescent center materialis high as shown in FIG. 19. Further, when portions related to carriertransport in both molecules are close to each other, an improvement incarrier mobility is expected.

By applying the above-described calculation results to thelight-emitting element in Embodiment 1, a light-emitting element inwhich excitation energy can be easily transferred from a host materialto a luminescent center material can be provided. In other words, in thelight-emitting element described in Embodiment 1, the direction of thetransition dipole moment of light emission of a host material ispreferably approximately in parallel to a monomolecular layer, andideally in parallel to the monomolecular layer. In addition, thedirection of the transition dipole moment of electronic transition of aluminescent center material is preferably close to in parallel to amonomolecular layer, and ideally in parallel to the monomolecular layer.

Conceptual views are shown in FIGS. 20A and 20B. Reference numerals 300,301, 350, and 351 denote the host material, the luminescent centermaterial, a transition dipole moment of light emission of the hostmaterial, and a transition dipole moment of absorption of theluminescent center material, respectively. Note that the direction ofthe transition dipole moment corresponds to the direction of arrows. Asshown in FIG. 20A, the transition dipole moment of the host material andthe transition dipole moment of the luminescent center material areideally in parallel to respective monomolecular layers. When thetransition dipole moments are in parallel to the monomolecular layers,the directions does not matter. When the monomolecular layers areprovided to have an angle to a substrate as shown in FIG. 20B, adistance for absorbing light emitted from the host material becomes longbut the transition efficiency is good (although which is not asfavorable as that of FIG. 20A) because the directions of polarization oflight emission are the same in the host material.

Further, since light is emitted to be perpendicular to the transitiondipole moment of light emission, it is preferable that the direction ofthe transition dipole moment 351 of electronic transition of theluminescent center material 301 be in parallel to a substrate 310 asmuch as possible as shown in FIG. 21, in order to extract light emittedfrom the luminescent center material effectively from the light-emittingelement. Thus, the light extraction efficiency can be improved and theexternal quantum efficiency is improved, whereby a light-emittingelement with higher luminous efficiency can be obtained. Note that theother EL layer or an electrode is provided between the luminescentcenter material 301 and the substrate 310.

(Embodiment 3)

In this embodiment, a specific structure of a light-emitting elementhaving the structure described in Embodiment 1 is described withreference to FIG. 3A.

The light-emitting element of the present invention includes a pluralityof layers between a pair of electrodes. The plurality of layers is astack of layers each formed of substances having a highcarrier-injection property or a high carrier-transport property so thata light-emitting region is formed away from the electrodes, that is,carriers are recombined in a portion away from the electrodes.

In this embodiment, the light emitting element includes a firstelectrode 102, a second electrode 104, and an EL layer 103 providedbetween the first electrode 102 and the second electrode 104. Note thatin description of this embodiment, the first electrode 102 functions asan anode and the second electrode 104 functions as a cathode. That is,in the following description, it is assumed that when voltage is appliedto the first electrode 102 and the second electrode 104 so that apotential of the first electrode 102 is higher than a potential of thesecond electrode 104, light is emitted.

A substrate 101 is used as a support of the light-emitting element. Forthe substrate 101, glass, plastic, metal, or the like can be used, forexample. Note that materials other than these can be used as long asthey can function as a support of a light-emitting element. Note thatwhen light emission from the light-emitting element is extracted outsidethrough the substrate 101, the substrate 101 is preferably alight-transmitting substrate. Further, in the case of using a substratethrough which water easily passes, such as a plastic substrate, it ispreferable to use the substrate over which a protective film with smallmoisture permeability is formed.

The first electrode 102 is preferably formed using any of metals,alloys, or conductive compounds, a mixture thereof, or the like with ahigh work function (specifically, a work function of greater than orequal to 4.0 eV is preferable). For example, indium oxide-tin oxide(ITO: indium tin oxide), indium oxide-tin oxide containing silicon orsilicon oxide, indium oxide-zinc oxide (IZO: indium zinc oxide), indiumoxide containing tungsten oxide and zinc oxide (IWZO), and the like aregiven. Such conductive metal oxide films are generally formed by asputtering method, but may also be formed by an ink-jet method, a spincoating method, or the like by application of a sol-gel method or thelike. For example, indium oxide-zinc oxide (IZO) can be formed by asputtering method using indium oxide to which zinc oxide of 1 wt % to 20wt % is added, as a target. Further, indium oxide including tungstenoxide and zinc oxide (IWZO) can be formed by a sputtering method usingindium oxide to which tungsten oxide of 0.5 wt % to 5 wt % and zincoxide of 0.1 wt % to 1 wt % are added, as a target. Besides, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), titanium(Ti), graphene, nitride of a metal material (e.g., titanium nitride),and the like are given.

In the case where a layer including the composite material described inEmbodiment 1 is used as a layer in contact with the first electrode 102,various metals, alloys, electrically conductive compounds, or a mixturethereof can be used for the first electrode 102 regardless of whetherthe work function is small or large. For example, aluminum (Al), silver(Ag), an alloy including aluminum (AlSi), or the like can be used.

Besides, an element belonging to Group 1 or Group 2 of the periodictable, which has a low work function, i.e., an alkali metal such aslithium (Li) or cesium (Cs); an alkaline earth metal such as magnesium(Mg), calcium (Ca), or strontium (Sr), an alloy including them (e.g.,MgAg or AlLi); a rare earth metal such as europium (Eu) or ytterbium(Yb), an alloy including them; or the like can be used. A film of analkali metal, an alkaline earth metal, or an alloy including thesemetals can be formed by a vacuum evaporation method. An alloy containingan alkali metal or an alkaline earth metal can be formed by a sputteringmethod. Further, a silver paste or the like can be formed by an ink-jetmethod or the like.

The EL layer 103 described in this embodiment includes a hole-injectionlayer 111, a hole-transport layer 112, a light-emitting layer 113, anelectron-transport layer 114, and an electron-injection layer 115. TheEL layer 103 includes at least a light-emitting layer, and there is noparticular limitation on a structure of the other stacked layers. Inother words, there is no particular limitation on the stacked structureof the EL layer 103. Layers formed of a substance with a highelectron-transport property, a substance with a high hole-transportproperty, a substance with a high electron-injection property, asubstance with a high hole-injection property, a bipolar substance (asubstance having high electron-transport and high hole-transportproperties), a substance with a high light-emitting property may becombined with the light-emitting layer 113 having the structuredescribed in Embodiment 1 as appropriate to form the EL layer 103. Forexample, the EL layer 103 may be formed by combining a hole-injectionlayer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, an electron-injection layer, and the like, asappropriate. Specific materials to form each of the layers are givenbelow.

The hole-injection layer 111 is a layer containing a substance with ahigh hole-injection property. As a substance with a high hole-injectionproperty, molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, or the like can be used. Besides, as a lowmolecular organic compound, the following compounds are given:phthalocyanine-based compounds such as phthalocyanine (abbreviation:H₂Pc), copper(II) phthalocyanine (abbreviation: CuPc), and vanadylphthalocyanine (abbreviation: VOPc); aromatic amine compounds such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Further, as the hole-injection layer 111, the composite materialdescribed in detail in Embodiment 1 can also be used, and a repeateddescription is omitted. The corresponding description in Embodiment 1 isto be referred to. Note that by using the composite material, a materialfor forming an electrode may be selected regardless of the work functionas described above. In other words, besides a material with a high workfunction, a material with a low work function may also be used as thefirst electrode 102. Such a composite material can be formed byco-deposition of a substance with a high hole-transport property and anacceptor substance.

The hole-transport layer 112 is a layer including a substance with ahigh hole-transport property. As the substance with a highhole-transport property, the following low molecular organic compoundcan be used: an aromatic amine compound such as NPB (or α-NPD), TPD,4,4′-bis[N-(9,9-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: m-MTDATA),N-[4-(9H-carbazol-9-yl)phenyl]-N-phenyl-spiro-9,9′-bifluoren-2-amine(abbreviation: YGASF),N,N′-bis[4-(9H-carbazol-9-yl)phenyl-N,N′-diphenylvinyl-4,4′-diamine(abbreviation: YGABP), 4-(9H-carbazol-9-yl)-2′-phenyltriphenylamine(abbreviation: o-YGA1BP), 4-(9H-carbazol-9-yl)-3′-phenyltriphenylamine(abbreviation: m-YGA1BP), 4-(9H-carbazol-9-yl)-4′-phenyltriphenylamine(abbreviation: p-YGA1BP), 1,3,5-tri(N-carbazolyl)benzene (abbreviation:TCzB), or 4,4′,4″-tris(N-carbazolyl)triphenylamine (abbreviation: TCTA).The substances given here are mainly substances each having a holemobility of higher than or equal to 10⁻⁶ cm²/Vs. However, any substanceother than the above substances may also be used as long as it is asubstance with a higher hole-transport property than anelectron-transport property. The layer including a substance with a highhole-transport property is not limited to a single layer, and a stackedlayer of two or more layers formed of the above materials may also beused.

Alternatively, for the hole-transport layer 112, a high molecularcompound such as PVK, PVTPA, PTPDMA, or Poly-TPD can be used.

The light-emitting layer 113 is a layer including a luminescent centermaterial formed of a substance having a high light-emitting property,and various materials can be used for the light-emitting layer 113. Asthe luminescent center material, for example, a fluorescent compoundexhibiting fluorescence or a phosphorescent compound exhibitingphosphorescence can be used.

Examples of a phosphorescent compound which can be used for thelight-emitting layer are given below. As a material for blue lightemission,bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)tetrakis(1-pyrazolyl)borate(abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: FIrpic),bis[2-(3′,5′bistrifluoroethylphenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: Ir(CF₃ ppy)₂(pic)),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]ridium(III)acetylacetonate(abbreviation: FIr(acac)), or the like is given. As a material for greenlight emission, tris(2-phenylpyridinato-N,C²′)iridium(III)(abbreviation: Ir(ppy)₃),bis[2-phenylpyridinato-N,C²′]iridium(III)acetylacetonate (abbreviation:Ir(ppy)₂(acac)),bis(1,2-diphenyl-1H-benzimidazolato)iridium(III)acetylacetonate(abbreviation: Ir(pbi)₂(acac)),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)), or the like is given. As a material for yellow lightemission, bis(2,4-diphenyl-1,3-oxazolato-N,C²′)iridium(III)acetylacetonate (abbreviation: Ir(dpo)₂(acac)),bis[2-(4′-perfluorophenylphenyl)pyridinato]iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)),bis(2-phenylbenzothiazolato-N,C²′)iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)), or the like is given. As a material fororange light emission, tris(2-phenylquinolinato-N,C²′)iridium(III)(abbreviation: Ir(pq)₃),bis(2-phenylquinolinato-N,C²′)iridium(III)acetylacetonate (abbreviation:Ir(pq)₂(acac)), or the like is given. As a material for red lightemission, an organometallic complex such asbis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C³′]iridium(III)acetylacetonate (abbreviation: Ir(btp)₂(acac)),bis(1-phenylisoquinolinato-N,C²′)iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP), or the like is given. In addition, a rare earthmetal complex such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), ortris[1-(2-thenyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen))performs light emission (electron transition between differentmultiplicities) from a rare earth metal ion; thus, such a rare earthmetal complex can be used as a phosphorescent compound.

Examples of fluorescent compounds that can be used for thelight-emitting layer are given below. Examples of materials for bluelight emission are as follows:N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthyrl)triphenylamine(abbreviation: 2YGAPPA);N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA); perylene; 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP);4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA); and the like. In addition, as a light-emittingmaterial for green light emission, the following can be used:N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA);N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA); N-(9,10-diphenyl-2-anthryl)-N,NN′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA);N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA);N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA); N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA); and the like. Further, examples of thematerials that emit yellow light include rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),and the like. Furthermore, examples of materials for red light emissioninclude N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine(abbreviation: p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fiuoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like.

The above-described luminescent center material and a host material aremanufactured to have the structure described in Embodiment 1.

Various materials can be used as the host material. Specifically, metalcomplex, heterocyclic compounds, and aromatic amine compounds are given,for example. As metal complexes, the following can be given:tris(8-quinolinolato)aluminum(III) (abbreviation: Alq);tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃);bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂);bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq); bis(8-quinolinolato)zinc(II) (abbreviation: Znq);bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO);bis[2-(2-benzothiazolyephenolato]zinc(II) (abbreviation: ZnBTZ); and thelike. As heterocyclic compounds, the following can be given:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7);3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ);2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI); bathophenanthroline (BPhen); bathocuproine(abbreviation: BCP);9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11); and the like. As aromatic amine compounds, the following can begiven: 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPBor α-NPD);N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD);4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]-1,1′-biphenyl(abbreviation: BSPB); and the like. In addition, condensed polycyclicaromatic compounds such as anthracene derivatives, phenanthrenederivatives, pyrene derivatives, chrysene derivatives, anddibenzo[g,p]chrysene derivatives are given. Specific examples of thecondensed polycyclic aromatic compound include 9,10-diphenylanthracene(abbreviation: DPAnth),N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetramine(abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3¹-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3), and thelike. It is preferable to use a substance with the lowest unoccupiedmolecular orbital (a LUMO level) higher than that of the luminescentcenter material and the highest occupied molecular orbital (a HOMOlevel) lower than that of the luminescent center material selected fromthe above substances and known substances. Note that in thisspecification, “the HOMO level or the LUMO level is high” means that theenergy level is high, while “the HOMO level or the LUMO level is low”means that the energy level is low. For example, it can be said that asubstance A having a HOMO level of −5.5 eV has a HOMO level which islower by 0.3 eV than that of a substance B having a HOMO level of −5.2eV, and has a HOMO level which is higher by 0.2 eV than that of asubstance C having a HOMO level of −5.7 eV.

When materials are selected from the host material and the luminescentcenter material with such a relation, preferably, materials havingsimilar skeletons are selected, excitation energy is smoothly given andreceived as described in Embodiment 1. Thus, it is possible tomanufacture a light-emitting element with high internal quantumefficiency and high luminous efficiency.

The electron-transport layer 114 is a layer containing a substance witha high electron-transport property. For example, as a low molecularorganic compound, metal complexes such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ) can beused. Further, the following heterocyclic compounds can be also used:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7);3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ01);2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI); bathophenanthroline (abbreviation: BPhen);bathocuproine (abbreviation: BCP); and the like. The substancesmentioned here are mainly ones that have an electron mobility higherthan or equal to 10⁻⁶ cm²/V·s. Note that the electron-transport layer114 may be formed of substances other than those described above as longas the substances have higher electron-transport properties thanhole-transport properties. Further, the electron-transport layer is notlimited to a single layer, and a stacked layer of two or more layersmade of the aforementioned substances may be used.

In addition, for the electron-transport layer 114, a high molecularcompound can be used. For example,poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py),poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy), or the like can be used.

The electron-injection layer 115 is a layer including a substance with ahigh electron-injection property. As the substance with a highelectron-injection property, an alkali metal, an alkaline earth metal,or a compound thereof such as lithium (Li), magnesium (Mg), lithiumfluoride (LiF), cesium fluoride (CsF), or calcium fluoride (CaF₂) can beused. For example, a layer of a material having an electron-transportproperty containing an alkali metal, an alkaline earth metal, or acompound thereof, such as Alq which contains magnesium (Mg), may beused. By using a layer of a substance having an electron-transportproperty containing an alkali metal or an alkaline earth metal as theelectron-injection layer, electron injection from the second electrode104 is performed efficiently, which is preferable.

As a substance for forming the second electrode 104, a metal, an alloy,an electrically conductive compound, a mixture thereof, or the like witha low work function (specifically, a work function of lower than orequal to 3.8 eV is preferable) can be used. As a specific example ofsuch a cathode material, an element belonging to Group 1 or 2 in theperiodic table, i.e., an alkali metal such as lithium (Li) or cesium(Cs), or an alkaline earth metal such as magnesium (Mg), calcium (Ca),or strontium (Sr); an alloy containing any of these (such as MgAg orAlLi); a rare earth metal such as europium (Eu) or ytterbium (Yb); analloy containing such a rare earth metal; or the like can be used. Afilm of an alkali metal, an alkaline earth metal, or an alloy includingthese metals can be formed by a vacuum evaporation method. An alloycontaining an alkali metal or an alkaline earth metal can be formed by asputtering method. Further, a silver paste or the like can be formed byan ink jet method or the like.

Further, by providing the electron-injection layer 115, which is a layerhaving a function of promoting injection of electrons, between thesecond electrode 104 and the electron-transport layer 114, the secondelectrode 104 can be formed using various conductive materials such asAl, Ag, ITO, or indium oxide-tin oxide containing silicon or siliconoxide, regardless of whether the work function is small or large. Filmsof these conductive materials can be formed by a sputtering method, anink-jet method, a spin coating method, or the like.

A variety of methods can be used for forming the EL layer regardless ofwhether it is a dry process or a wet process. For example, a vacuumevaporation method, an ink-jet method, a spin coating method or the likemay be used. A different foimation method may be employed for eachelectrode or each layer. The organic semiconductor material described inEmbodiment 1 exhibits a good sublimation property; thus, a favorablefilm can be formed by an evaporation method by using the organicsemiconductor material.

For example, the EL layer may be formed using a high molecular compoundby a wet process. Alternatively, the EL layer can be formed using a lowmolecular organic compound by a wet process. Further alternatively, theEL layer may be formed using a low molecular organic compound by a dryprocess such as a vacuum evaporation method.

In addition, the electrode may be formed by a wet process using asol-gel method, or by a wet process using paste of a metal material.Alternatively, the electrode may be foamed by a dry process such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above structure, current flowsdue to a potential difference generated between the first electrode 102and the second electrode 104 and holes and electrons are recombined inthe EL layer 103; thus, light is emitted.

The emitted light is extracted through one or both of the firstelectrode 102 and the second electrode 104. Accordingly, one or both ofthe first electrode 102 and the second electrode 104 is/are an electrodehaving a light-transmitting property. For example, when only the firstelectrode 102 has a light-transmitting property, light emission isextracted from the substrate side through the first electrode 102.Meanwhile, when only the second electrode 104 has a light-transmittingproperty, light emission is extracted from the side opposite to thesubstrate side through the second electrode 104. In the case where eachof the first electrode 102 and the second electrode 104 has alight-transmitting property, light emission is extracted from both ofthe substrate side and the side opposite to the substrate through thefirst electrode 102 and the second electrode 104.

The structure of the layers provided between the first electrode 102 andthe second electrode 104 is not limited to the above structure. Anystructure other than the above structure is included in the category ofthis embodiment as long as a light-emitting region for recombination ofholes and electrons is positioned away from the first electrode 102 andthe second electrode 104 to prevent quenching caused by proximity of thelight-emitting region to metal, and a light-emitting layer having thestructure described in Embodiment 1 is provided.

In other words, there is no particular limitation on the stackedstructure of the layers; layers formed of a substance having a highelectron-transport property, a substance having a high hole-transportproperty, a substance having a high electron-injection property, asubstance having a high hole-injection property, a bipolar substance (asubstance having high electron-transport and hole-transport properties),and the like may be combined with a light-emitting layer having thestructure described in Embodiment 1 as appropriate to form the stackedstructure.

For example, as illustrated in FIG. 3B, a structure may be employed inwhich the second electrode 104 functioning as a cathode, the EL layer103, and the first electrode 102 functioning as an anode are stacked inthis order over the substrate 101. In FIG. 3B, a structure in which theelectron-injection layer 115, the electron-transport layer 114, thelight-emitting layer 113, the hole-transport layer 112, and thehole-injection layer 111 are stacked in this order over the secondelectrode 104 is employed.

In this embodiment, the light-emitting element is formed over asubstrate including glass, plastic, or the like. By forming a pluralityof such light emitting elements over one substrate, a passive matrixlight emitting device can be manufactured. In addition, a thin filmtransistor (TFT) may be formed over a substrate including glass,plastic, or the like and a light-emitting element may be manufacturedover an electrode that is electrically connected to the TFT. In thismanner, an active matrix light-emitting device in which the TFT controlsthe drive of the light-emitting element can be manufactured. Note thatthe structure of the TFT is not particularly limited. A staggered TFT oran inverted staggered TFT may be used. Further, a driver circuit formedover a TFT substrate may include both n-channel and p-channel TFTs oronly one of n-channel and p-channel TFTs. There is no particularlimitation on crystallinity of a semiconductor film used for the TFT. Anamorphous semiconductor film may be used, or a crystalline semiconductorfilm may be used. Further, a single crystalline semiconductor film maybe used. The single crystalline semiconductor film can be fanned by aSmart Cut (registered trademark) method or the like.

Note that this embodiment can be freely combined with the otherembodiments.

(Embodiment 4)

In this embodiment, a mode of a light-emitting element of the presentinvention having a structure in which a plurality of light-emittingunits is stacked (hereinafter, referred to as a stacked-type element)will be described with reference to FIG. 4. The light-emitting elementis a stacked-type light-emitting element including a plurality oflight-emitting units between a first electrode and a second electrode. Astructure of each of the light-emitting units can be similar to thestructure of the EL layer 103 described in Embodiment 3. In other words,it can be said that the light-emitting element described in Embodiment 3is a light-emitting element including one light-emitting unit. In thisembodiment, a light-emitting element including a plurality oflight-emitting units will be described.

In FIG. 4, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502. A charge generating layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.Electrodes similar to those in Embodiment 3 can be applied to the firstelectrode 501 and the second electrode 502. In addition, the firstlight-emitting unit 511 and the second light-emitting unit 512 may havethe same structure or different structures, and at least one of thefirst light-emitting unit 511 and the second light-emitting unit 512 hasa structure similar to the EL layer 103 described in Embodiment 3.

The charge generating layer 513 is a layer that injects electrons into alight-emitting unit on one side and injects holes into a light-emittingunit on the other side when a voltage is applied to the first electrode501 and the second electrode 502. The charge generating layer 513 mayhave a single-layer structure or a stacked structure. As a stackedstructure of plural layers, a structure in which a hole-injection layerand an electron-injection layer are stacked is preferable.

As the hole-injection layer, a semiconductor or an insulator, such asmolybdenum oxide, vanadium oxide, rhenium oxide, or ruthenium oxide, canbe used. Alternatively, the hole-injection layer may have a structure inwhich an acceptor substance is added into a substance having a highhole-transport property. The layer containing a substance having a highhole-transport property and an acceptor substance is fowled using thecomposite material described in Embodiment 3 and includes, as anacceptor substance, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ) or metal oxide such as vanadium oxide,molybdenum oxide, or tungsten oxide. As the substance having a highhole-transport property, various compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (such as oligomers, dendrimers, and polymers) can beused. Note that a substance having a hole mobility higher than or equalto 10⁻⁶ cm²/Vs is preferably used as the substance having a highhole-transport property. However, any substance other than the abovesubstances may also be used as long as it is a substance whosehole-transport property is higher than the electron-transport property.Since the composite material including the substance having a highhole-transport property and the acceptor substance is superior incarrier-injection property and carrier-transport property, low voltagedriving and low current driving can be realized.

As the electron-injection layer, an insulator or a semiconductor, suchas lithium oxide, lithium fluoride, or cesium carbonate, can be used.Alternatively, the electron-injection layer may have a structure inwhich a donor substance is added to a substance having a highelectron-transport property. As the donor substance, an alkali metal, analkaline-earth metal, a rare-earth metal, a metal that belongs to Group13 of the periodic table, or an oxide or carbonate thereof can be used.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used as the donor substance. As thesubstance having a high electron-transport property, the materialsdescribed in Embodiment 3 can be used. Note that a substance having ahole mobility higher than or equal to 10⁻⁶ cm²/Vs or more is preferablyemployed as the substance having a high hole-transport property. Notethat any substance that has a higher electron-transport property than ahole-transport property may be used other than the above substances.Since the composite material of the substance having a highhole-transport property and the donor substance has an excellentcarrier-injection property and an excellent carrier-transport property,low voltage driving and low current driving can be realized.

Further, the electrode materials described in Embodiment 2 can be usedfor the charge generating layer 513. For example, the charge generatinglayer 513 may be formed with a combination of a layer including asubstance having a high hole-transport property and a metal oxide with atransparent conductive film. It is preferable that the charge generatinglayer 513 be a highly light-transmitting layer in view of lightextraction efficiency.

In any case, the charge generating layer 513, which is interposedbetween the first light-emitting unit 511 and the second light-emittingunit 512, is acceptable as long as electrons are injected to alight-emitting unit on one side and holes are injected to alight-emitting unit on the other side when a voltage is applied to thefirst electrode 501 and the second electrode 502. For example, anystructure is acceptable for the charge generating layer 513 as long asthe charge generating layer 513 injects electrons and holes into thefirst light-emitting unit 511 and the second light-emitting unit 512,respectively when applying a voltage so that a potential of the firstelectrode becomes higher than a potential of the second electrode.

Although this embodiment describes the light emitting element having twolight emitting units, the present invention can be similarly applied toa light emitting element in which three or more light emitting units arestacked. By arranging a plurality of light-emitting units between a pairof electrodes so as to be partitioned by a charge-generating layer as inthe light-emitting element of this embodiment, the element can performlight emission in a high luminance region while keeping a currentdensity low, whereby the element can have long life. When thelight-emitting element is applied for illumination, voltage drop due toresistance of an electrode material can be reduced, thereby achievinghomogeneous light emission in a large area. Moreover, a light-emittingdevice of low power consumption, which can be driven at low voltage, canbe achieved.

When the light-emitting units are foiined to emit different colors fromeach other, the light-emitting unit as a whole can provide lightemission of a desired color. For example, in a light-emitting elementhaving two light-emitting units, the emission colors of the firstlight-emitting unit and the second light-emitting unit are madecomplementary, so that the light-emitting element which emits whitelight as the whole element can be obtained. Note that the word“complementary” means color relationship in which an achromatic color isobtained when colors are mixed. That is, when light emitted fromsubstances which emit light of complementary colors are mixed, whitelight emission can be obtained. This can be applied to a light-emittingelement having three light-emitting units in a similar manner. Forexample, when the emission color of the first light-emitting unit is redlight, the emission color of the second light-emitting unit is greenlight, and the emission color of the third light-emitting unit is bluelight, the light-emitting element as a whole can provide white coloremission.

Note that this embodiment can be freely combined with the otherembodiments.

(Embodiment 5)

In this embodiment, a light-emitting device having the light-emittingelement of the present invention will be described.

In this embodiment, a light-emitting device having the light-emittingelement of the present invention in a pixel portion will be describedwith reference to FIGS. 5A and 5B. FIG. 5A is a top view of thelight-emitting device, and FIG. 5B is a cross-sectional view taken alonglines A-A′ and B-B′ of FIG. 5A. The light-emitting device includes adriver circuit portion (source-side driver circuit) 601, a pixel portion602, and a driver circuit portion (gate-side driver circuit) 603 whichare illustrated with dotted lines. These units control light emission ofthe light-emitting element. Further, a reference numeral 604 denotes asealing substrate; 605, a sealing material; and 607, a space surroundedby the sealing material 605.

Reference numeral 608 denotes a wiring for transmitting signals to beinputted to the source side driver circuit portion 601 and the gate sidedriver circuit portion 603 and receiving signals such as a video signal,a clock signal, a start signal, and a reset signal from an FPC (flexibleprinted circuit) 609 serving as an external input terminal. Althoughonly the FPC is illustrated here, a printed wiring board (PWB) may beattached to the FPC. The light-emitting device in this specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.5B. The driver circuit portion and the pixel portion are formed over anelement substrate 610. Here, the source side driver circuit portion 601,which is the driver circuit portion, and one pixel of the pixel portion602 are shown.

In the source side driver circuit portion 601, a CMOS circuit is formedin which an n-channel TFT 623 and a p-channel TFT 624 are combined. Sucha driver circuit may be formed by using various circuits such as a CMOScircuit, a PMOS circuit, or an NMOS circuit. In this embodiment, adriver-integrated type in which a driver circuit is formed over asubstrate provided with a pixel portion is described; however, thepresent embodiment is not necessarily limited to this type, and thedriver circuit can be formed outside the substrate.

The pixel portion 602 includes a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 connected electrically with a drain of the current controlling TFT612. An insulator 614 is formed to cover an end portion of the firstelectrode 613. Here, the insulator 614 is formed using a positive typephotosensitive acrylic resin film.

In order to improve the coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case of using positive photosensitive acrylic for thematerial of the insulator 614, it is preferable that only the upper endportion of the insulator 614 has a curved surface with a radius ofcurvature of 0.2 μm to 3 μm. As the insulator 614, either a negativetype which becomes insoluble in etchant by light irradiation or apositive type which becomes soluble in etchant by light irradiation canbe used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, the first electrode 613 can be formed using variousmetals, alloys, electrically conductive compounds, or mixture thereof.If the first electrode 613 is used as an anode, it is preferable to use,among those materials, a metal, an alloy, an electrically conductivecompound, a mixture thereof, or the like with a high work function(preferably, a work function higher than or equal to 4.0 eV). Forexample, the first electrode 613 can be formed using a single-layer filmof an indium tin oxide film containing silicon, an indium zinc oxidefilm, a titanium nitride film, a chromium film, a tungsten film, a Znfilm, a Pt film, or the like; or a stacked film such as a stack of atitanium nitride film and a film containing aluminum as its maincomponent or a three-layer structure of a titanium nitride film, a filmcontaining aluminum as its main component, and a titanium nitride film.The stacked structure achieves to have low wiring resistance, favorableohmic contact, and a function as an anode.

The EL layer 616 is fondled by various methods such as an evaporationmethod using an evaporation mask, an inkjet method, a spin coatingmethod, or the like. The EL layer 616 includes the organic semiconductormaterial described in Embodiment 1. Low molecular compounds, highmolecular compounds, oligomers, or dendrimers may be used as a materialfor the EL layer 616. As the material for the EL layer, not only anorganic compound but also an inorganic compound may be used.

As the material for the second electrode 617, various types of metals,alloys, electrically conductive compounds, mixtures of these, or thelike can be used. If the second electrode is used as a cathode, it ispreferable to use a metal, an alloy, an electrically conductivecompound, a mixture thereof, or the like with a low work function(preferably, a work function of 3.8 eV or lower) among such materials.As an example, an element belonging to Group 1 or Group 2 in theperiodic table, i.e., an alkali metal such as lithium (Li) or cesium(Cs), an alkaline earth metal such as magnesium (Mg), calcium (Ca), orstrontium (Sr), or an alloy containing any of these (such as MgAg orAlLi); and the like are given. In the case where light generated in theEL layer 616 is transmitted through the second electrode 617, the secondelectrode 617 can be formed using a stack of a metal thin film and atransparent conductive film (indium tin oxide (ITO), indium tin oxidecontaining silicon or silicon oxide, indium zinc oxide (IZO), indiumoxide containing tungsten oxide and zinc oxide (IWZO), or the like).

Further, the sealing substrate 604 is attached to the element substrate610 with the sealing material 605, so that a light-emitting element 618is provided in the space 607 surrounded by the element substrate 610,the sealing substrate 604, and the sealing material 605. Note that thespace 607 is filled with a filler in some cases. There are cases wherethe space 607 may be filled with an inert gas (such as nitrogen orargon), or where the space 607 may be filled with a material similar tothe sealant 605.

An epoxy based resin is preferably used for the sealing material 605. Amaterial used for these is desirably a material which does not transmitmoisture or oxygen as possible. As a material for the sealing substrate604, a plastic substrate made of Fiberglass-Reinforced Plastics (FRP),polyvinyl fluoride (PVF), polyester, acrylic, or the like can be usedbesides a glass substrate or a quartz substrate.

As described above, the light-emitting device having the light-emittingelement of the present invention can be obtained.

The light-emitting device of the present invention includes thelight-emitting element described in any of Embodiments 1 to 3. Thelight-emitting element described in any of Embodiments 1 to 3 has highluminous efficiency and a low driving voltage. Therefore, alight-emitting device which can emit light with high luminance can beobtained. Further, a light-emitting device with low power consumptioncan be obtained.

As described above, an active-matrix light-emitting device that controlsdriving of a light-emitting element with a transistor is described inthis embodiment; however, a passive-matrix light-emitting device may beused. FIGS. 6A and 6B show a passive matrix light-emitting devicemanufactured by application of the present invention. FIG. 6A is aperspective view of the light-emitting device, and FIG. 6B is across-sectional view taken along a line X-Y of FIG. 6A. In FIGS. 6A and6B, over a substrate 951, an EL layer 955 is provided between anelectrode 952 and an electrode 956. An edge portion of the electrode 952is covered with an insulating layer 953. A partition layer 954 isprovided over the insulating layer 953. The sidewalls of the partitionlayer 954 are aslope such that the distance between both sidewalls isgradually narrowed toward the surface of the substrate. That is, a crosssection taken along the direction of the short side of the partitionwall layer 954 is trapezoidal, and the lower side (a side which is inthe same direction as a plane direction of the insulating layer 953 andin contact with the insulating layer 953) is shorter than the upper side(a side which is in the same direction as the plane direction of theinsulating layer 953 and not in contact with the insulating layer 953).A cathode can be patterned by providing the partition layer 954 in thismanner. In addition, in a passive matrix light-emitting device, alight-emitting device with low power consumption can be obtained byincluding a light-emitting element with high emission efficiency and lowdriving voltage according to the present invention.

Note that this embodiment can be freely combined with the otherembodiments.

(Embodiment 6)

In this embodiment, electronic devices of the present invention eachincluding the light-emitting device described in Embodiment 4 in a partwill be described. The electronic devices of the present invention eachhave the light-emitting element described in any of Embodiments 1 to 3,and thus have a display portion with low power consumption.

As the electronic device manufactured by using the light-emitting deviceof the present invention, video cameras or digital cameras, goggle-typedisplays, navigation systems, audio reproducing devices (such as caraudio components or an audio components), computers, game machines,portable information terminals (mobile computers, cellular phones,mobile game machines, or electronic books), image reproducing devicesequipped with a recording medium (specifically, devices equipped with adisplay device for reproducing a recording medium such as digitalversatile disk (DVD) and displaying the image), and the like are given.Specific examples of these electronic devices are shown in FIGS. 6A and6B, FIGS. 7A to 7D, FIGS. 8A to 8C, FIG. 9, FIG. 10, FIG. 11, FIG. 12,and FIG. 13.

FIG. 7A shows a television device of this embodiment, which includes ahousing 9101, a support base 9102, a display portion 9103, a speakerportion 9104, a video input terminal 9105, and the like. In the displayportion 9103 of this television device, light-emitting elements similarto those described in any of Embodiments 1 to 3 are arranged in matrix.The light-emitting elements have a characteristic of high luminousefficiency and low power consumption. In addition, the light-emittingelements also have a characteristic of low driving voltage. The displayportion 9103 including the light-emitting elements has similarcharacteristics; therefore, the television device has low powerconsumption. Such characteristics can dramatically reduce or downsizepower supply circuits in the television device, whereby the housing 9101and the support base 9102 can be reduced in size and weight. In thetelevision device of this embodiment, reduction in power consumption andreduction in size and weight are achieved; thus, a product suitable forliving environment can be provided.

FIG. 7B shows a computer of this embodiment, which includes a main body9201, a housing 9202, a display portion 9203, a keyboard 9204, anexternal connection port 9205, a pointing device 9206, and the like. Inthe display portion 9203 of this computer, light-emitting elementssimilar to those described in any of Embodiments 1 to 3 are arranged inmatrix. The light-emitting elements have a characteristic of highluminous efficiency and low power consumption. In addition, thelight-emitting elements also have a characteristic of low drivingvoltage. The display portion 9203 including the light-emitting elementshas similar characteristics; therefore, the computer consumes low power.Such characteristics can dramatically reduce or downsize power supplycircuits in the computer, whereby the main body 9201 and the housing9202 can be reduced in size and weight. In the computer of thisembodiment, reduction in power consumption and reduction in size andweight are achieved; thus, a product suitable for environment can beprovided.

FIG. 7C shows a camera of this embodiment, which includes a main body9301, a display portion 9302, a housing 9303, an external connectionport 9304, a remote control receiving portion 9305, an image receivingportion 9306, a battery 9307, audio input portion 9308, operation keys9309, an eyepiece portion 9310, and the like. In the display portion9302 of this camera, light-emitting elements similar to those describedin any of Embodiments 1 to 3 are arranged in matrix. The light-emittingelements have a characteristic of high luminous efficiency and low powerconsumption. In addition, the light-emitting elements also have acharacteristic of low driving voltage. The display portion 9302including such light-emitting elements has similar characteristics;therefore, this camera consumes low power. Such characteristics candramatically reduce or downsize power supply circuits in the camera,whereby the main body 9301 can be reduced in size and weight. In thecamera of this embodiment, reduction in power consumption and reductionin size and weight are achieved; thus, a product suitable for beingcarried around can be provided.

FIG. 7D shows a mobile phone of this embodiment, which includes a mainbody 9401, a housing 9402, a display portion 9403, an audio inputportion 9404, an audio output portion 9405, operation keys 9406, anexternal connection port 9407, an antenna 9408, and the like. In thedisplay portion 9403 of this mobile phone, light-emitting elementssimilar to those described in any of Embodiments 1 to 3 are arranged inmatrix. The light-emitting elements have a characteristic of highluminous efficiency and low power consumption. In addition, thelight-emitting elements also have a characteristic of low drivingvoltage. The display portion 9403 including the light-emitting elementshas similar characteristics; therefore, the mobile phone consumes lowpower. Such characteristics can dramatically reduce or downsize powersupply circuits in the cellular phone, whereby the main body 9401 andthe housing 9402 can be reduced in size and weight. In the mobile phoneof this embodiment, reduction in power consumption and reduction in sizeand weight are achieved; thus, a product suitable for being carriedaround can be provided.

FIGS. 8A to 8C show an example of a mobile phone having a differentstructure shown in FIG. 7D. FIG. 8A is a front view, FIG. 8B is a rearview, and FIG. 8C is a development view. The mobile phone shown in FIGS.8A to 8C is a so-called smartphone having a function of a phone and afunction of a portable information terminal, incorporating a computer,and conducting a variety of data processing in addition to voice calls.

The mobile phone shown in FIGS. 8A to 8C has two housings: a housing1001 and a housing 1002. The housing 1001 includes a display portion1101, a speaker 1102, a microphone 1103, operation keys 1104, a pointingdevice 1105, a camera lens 1106, an external connection terminal 1107,an earphone terminal 1108, and the like, while the housing 1002 includesa keyboard 1201, an external memory slot 1202, a camera lens 1203, alight 1204, and the like. In addition, an antenna is incorporated in thehousing 1001.

In addition to the above-described structure, the mobile phone mayincorporate a wireless IC chip, a small size memory device, or the like.

The light-emitting device described in Embodiment 4 can be incorporatedin the display portion 1101, and a display orientation can beappropriately changed depending on a usage pattern. Because the cameralens 1106 is provided in the same plane as the display portion 1101, themobile phone can be used as a videophone. Further, a still image and amoving image can be taken with the camera lens 1203 and the light 1204by using the display portion 1101 as a viewfinder. The speaker 1102 andthe microphone 1103 can be used for video calls, recording, reproducing,and the like without being limited to voice calls. With the use of theoperation keys 1104, making and receiving calls, inputting simpleinformation such as e-mail or the like, scrolling the screen, moving thecursor, and the like are possible. Further, the housing 1001 and thehousing 1002 in FIG. 8A, which are overlapped with each other, are slidas shown in FIG. 8C, and can be used as a portable information terminal.In that case, smooth operation can be conducted using the keyboard 1201and the pointing device 1105. The external connection terminal 1107 canbe connected to an AC adaptor and various types of cables such as a USBcable, and charging and data communication with a computer or the likeare possible. Furthermore, a large amount of data can be stored andmoved by inserting a recording medium into the external memory slot1202.

The mobile phone may be equipped with an infrared communicationfunction, a television receiving function, and the like, in addition tothe above-described functions.

FIG. 9 is an audio reproducing device, specifically, a car audio stereo,which includes amain body 701, a display portion 702, and operationswitches 703 and 704. The display portion 702 can be realized using thelight-emitting device (passive-matrix type or active-matrix type)described in Embodiment 4. Further, this display portion 702 may beformed using a segment type light-emitting device. In any case, the useof a light-emitting element of the present invention makes it possibleto form a bright display portion while achieving low power consumption,with the use of a vehicle power source (12 V to 42 V). Although anin-car audio system is described in this embodiment, the presentinvention may be used for a portable audio device or an audio device forhousehold use.

FIG. 10 shows a digital player as an example of an audio reproducingdevice. The digital player shown in FIG. 10 includes a main body 710, adisplay portion 711, a memory portion 712, an operation portion 713, apair of earphones 714, and the like. Note that a pair of headphones or apair of wireless earphones can be used instead of the pair of earphones714. The display portion 711 can be realized using the (passive matrixor active matrix) light-emitting device described in Embodiment 4.Further, the display portion 711 may be formed using a segment typelight-emitting device. In any case, the use of the light-emittingelement of the present invention makes it possible to form a brightdisplay portion which can display images even when using a secondarybattery (a nickel-hydrogen battery or the like) while achieving lowpower consumption. As the memory portion 712, a hard disk or anonvolatile memory is used. For example, a NAND type nonvolatile memorywith a recording capacity of 20 gigabytes (GB) to 200 gigabytes (GB) isused and the operation portion 713 is operated, whereby an image or asound (for example, music) can be recorded and reproduced. Note thatpower consumption can be reduced by displaying white characters againsta black background in the display portions 702 and 711. This isparticularly effective in a portable audio device.

As described above, an application range of the light-emitting devicemanufactured by employing the present invention is quite wide, and thislight-emitting device can be applied to electronic devices of variousfields. By employing the present invention, an electronic device havinga display portion with low power consumption can be manufactured.

Further, a light-emitting device employing the present inventionincludes the light-emitting element described in any of Embodiments 1 to3 having high luminous efficiency and can be used as a lighting device.The light-emitting device employing the present invention can emit lightwith high luminance and is preferably used as a lighting device. Oneembodiment of using the light-emitting element employing the presentinvention as a lighting device is described with reference to FIG. 11.

FIG. 11 shows a liquid crystal display device using the light-emittingdevice of the present invention as backlight, as an example of anelectronic device that is a lighting device using the light-emittingelement of the present invention. The liquid crystal display deviceshown in FIG. 11 includes a housing 901, a liquid crystal layer 902, abacklight 903, and a housing 904, and the liquid crystal layer 902 isconnected to a driver IC 905. The light-emitting device to which thepresent invention is applied is used as the backlight 903, and a currentis supplied through a terminal 906.

Because the light-emitting device of the present invention is thin andconsumes low power, reduction in thickness and low power consumption ofa liquid crystal display device is possible by using a light-emittingdevice of the present invention as a backlight of the liquid crystaldisplay device. Moreover, a light-emitting device according to thepresent invention is a plane-emission lighting device and can beincreased in size. Thus, it becomes possible to increase the size of thebacklight and also a liquid crystal display device.

FIG. 12 shows an example of using a light-emitting device according tothe present invention as a desk lamp which is a lighting device. Thedesk lamp shown in FIG. 12 includes a housing 2001 and a light source2002, and a light-emitting device according to the present invention isused as the light source 2002. Because a light-emitting device of thepresent invention consumes low power, the desk lamp also consumes lowpower.

FIG. 13 shows an example in which a light-emitting device to which alight-emitting element of the present invention is applied is used asinterior lighting devices 3001 and 3003. Because a light-emitting deviceaccording to the present invention can be increased in size, it can beused as a large-area lighting device. In addition, since thelight-emitting element according to the present invention has highluminous efficiency and a light-emitting device using the light-emittingelement has low power consumption, it can be used for a lighting devicehaving low power consumption. A television device 3002 of the presentinvention as described with FIG. 7A can be placed in a room in which thelight-emitting device of the present invention is used as the interiorlighting device 3001, where public broadcasting and movies can beenjoyed. In such a case, since both devices consume low power,environmental load can be reduced.

Note that this embodiment can be freely combined with the otherembodiments.

This application is based on Japanese Patent Application Ser. No.2010-261192 filed with Japan Patent Office on Nov. 24, 2010, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting element comprising: a layerincluding an organic compound between a pair of electrodes, wherein thelayer comprises at least: a first monomolecular layer comprising aluminescent center material having a fluorescent light-emittingproperty; and a second monomolecular layer comprising a host materialhaving a carrier-transport property and a band gap larger than a bandgap of the luminescent center material, and wherein a surface of thesecond monomolecular layer is in contact with a surface of the firstmonomolecular layer, wherein a transition dipole moment of theluminescent center material is parallel to a top surface of a substrate,and wherein a transition dipole moment of the host material is parallelto the top surface of the substrate.
 2. The light-emitting elementaccording to claim 1, wherein a first skeleton of the host material isadjacent to a second skeleton of the luminescent center material,wherein the first skeleton contributes to acceptance of an excitationenergy, and wherein the second skeleton contributes to donation of anexcitation energy.
 3. The light-emitting element according to claim 1,wherein a first skeleton of the host material is adjacent to a secondskeleton of the luminescent center material, wherein the first skeletoncontributes to acceptance of a carrier, and wherein the second skeletoncontributes to donation of a carrier.
 4. The light-emitting elementaccording to claim 1, further comprising: a second layer consisting of acomposite material in which an acceptor substance is included in asubstance having a high hole-transport property, between the pair ofelectrodes, and wherein the second monomolecular layer and the secondlayer share an interface which is different from an interface shared bythe first monomolecular layer and the second monomolecular layer.
 5. Thelight-emitting element according to claim 4, wherein a plurality ofstacks is stacked, each of stacks comprising: the second layer; thefirst monomolecular layer; and the second monomolecular layer.
 6. Thelight-emitting element according to claim 5, wherein a total thicknessof the plurality of stacks is greater than or equal to 30 nm and lessthan or equal to 200 nm.
 7. A light-emitting device comprising: thelight-emitting element according to claim 1; and a control circuitcapable of controlling the light-emitting element.
 8. A lighting devicecomprising: the light-emitting element according to claim 1; and acontrol circuit capable of controlling the light-emitting element.
 9. Anelectronic device comprising the light-emitting device according toclaim
 7. 10. An electronic device comprising the lighting deviceaccording to claim
 8. 11. A light-emitting element comprising: a layerincluding an organic compound between a pair of electrodes, wherein thelayer comprises at least: a first monomolecular layer comprising aluminescent center material having a phosphorescent light-emittingproperty; and a second monomolecular layer comprising a host materialhaving a carrier-transport property and a triplet excited energy largerthan a triplet excited energy of the luminescent center material, andwherein a surface of the second monomolecular layer is in contact with asurface of the first monomolecular layer, wherein a transition dipolemoment of the luminescent center material is parallel to a top surfaceof a substrate, and wherein a transition dipole moment of the hostmaterial is parallel to the top surface of the substrate.
 12. Thelight-emitting element according to claim 11, wherein a first skeletonof the host material is adjacent to a second skeleton of the luminescentcenter material, wherein the first skeleton contributes to acceptance ofan excitation energy, and wherein the second skeleton contributes todonation of an excitation energy.
 13. The light-emitting elementaccording to claim 11, wherein a first skeleton of the host material isadjacent to a second skeleton of the luminescent center material,wherein the first skeleton contributes to acceptance of a carrier, andwherein the second skeleton contributes to donation of a carrier. 14.The light-emitting element according to claim 11, further comprising: asecond layer consisting of a composite material in which an acceptorsubstance is included in a substance having a high hole-transportproperty, between the pair of electrodes, and wherein the secondmonomolecular layer and the second layer share an interface which isdifferent from an interface shared by the first monomolecular layer andthe second monomolecular layer.
 15. The light-emitting element accordingto claim 14, wherein a plurality of stacks is stacked, each of stackscomprising: the second layer; the first monomolecular layer; and thesecond monomolecular layer.
 16. The light-emitting element according toclaim 15, wherein a total thickness of the plurality of stacks isgreater than or equal to 30 nm and less than or equal to 200 nm.
 17. Alight-emitting device comprising: the light-emitting element accordingto claim 11; and a control circuit capable of controlling thelight-emitting element.
 18. A lighting device comprising: thelight-emitting element according to claim 11; and a control circuitcapable of controlling the light-emitting element.
 19. An electronicdevice comprising the light-emitting device according to claim
 17. 20.An electronic device comprising the lighting device according to claim18.